1. Field of the Invention
The present invention relates to an organic coated steel which is excellent in rust preventive and corrosion preventive performances and a rust preventing method for metal, specifically to an organic coated steel which has a strong adhesive force between a polymer resin and the steel product and which is excellent in rust preventive and corrosion preventive performances and a rust preventing method in which a rust preventing and corrosion preventing effect can be maintained even under severe environment.
2. Description of the Related Arts
A steel product coated on a surface thereof with a polymer resin is excellent in decorativeness and functionality as compared with a painted steel product and can be expected from the viewpoint of environmental protection, and therefore it is used for building materials for houses, electric appliances and can materials.
In particular, a steel product coated on a surface thereof with a polyolefin resin (hereinafter referred to as a polyolefin-coated steel product) is excellent in a corrosion preventive performance over a long period of time, so that it is used for steel products for building materials used in the sea bottom, an extremely cold area and a tropical area and steel pipes for pipelines which transport crude oil, heavy oil and natural gas in addition to the uses of steel pipes, steel pipe piles, steel plates and the like. However, a transported fluid temperature in a pipeline is elevated because of deepening of oil wells and turning into heavier oil, and a steel product is required to be improved in a corrosion preventive performance under a wide temperature environment. In addition thereto, cathode peeling caused by excess protective current is a problem under environment in which electric protection is used in combination, and therefore an improvement in a cathode peeling resistance at high temperature is a subject. Further, in respect to electric protection, a steel product is required to be further improved in rust preventive and corrosion preventive performances in order to reduce an amount of electricity used for corrosion prevention.
There is conventional methods regarding a polyolefin-coated steel product for elevating a corrosion preventive performance of a steel product by providing chromate treatment between the steel product and a modified polyolefin adhesive layer or by allowing an epoxy resin to intervene therebetween. In these methods, the satisfactory performance is obtained in water at 60° C. or lower, but the satisfactory performance is not obtained in a cathode peeling resistance in water exceeding 60° C. Further, the adhesive property between a polyolefin resin layer and a steel product after hot water treatment is notably reduced, and therefore it is difficult to maintain the corrosion preventive performance over a long period of time.
Japanese Patent Application Laid-Open No. 170433/1999 proposed a technique for solving the problem described above that is making a steel product which surface is composed of a chromate layer, an epoxy resin layer formed by blending a specific epoxy resin, specific amines and a specific phenol resin as an organic filler, a modified polyolefin adhesive layer and a polyolefin resin layer to thereby provide a polyolefin-coated steel product. The product maintains well an adhesive property between the polyolefin resin layer under wide temperature environment and which is excellent in a warm water resistance and a cathode peeling resistance. However, the rust preventive and corrosion preventive performances are required to be further elevated in order to reduce an amount of electricity used in electric protection which is used in combination when used over a long period of time under humid and hot environment as is the case with, for example, steel pipes for a pipeline. Also, in uses of building materials for houses, electric appliances and can materials in which electric protection is not used in combination, a steel product is required to be further improved in rust preventive and corrosion preventive performances for the purpose of extending the use life of a steel product itself.
Also, steel plates and steel products obtained by subjecting the surface of a zinc base plated steel plate or an aluminum base plated steel plate to chromate processing with a processing solution comprising chromic acid, bichromic acid or salts thereof as a principal component for the purpose of elevating the corrosion preventive (rust preventive) property are widely used for steel plates and steel products which are widely used for home electric products, automobiles and building materials. This method is an economical method which provides an excellent corrosion preventive property and which can relatively readily be carried out.
On the other hand, many processing techniques which do not depend on chromate processing are proposed in order to prevent rust from being generated on various steel plates. A method in which tannic acid is used is proposed in, for example, Japanese Patent Application Laid-Open No. 71233/1976, and a method in which a mixed composition of an aqueous resin and polyhydric phenol carboxylic acid is used is proposed in Japanese Patent Application Laid-Open No. 325760/1996. In these methods, however, the barrier property to a corrosive factor such as oxygen and water is not satisfactory, and the corrosion preventive property over a long period of time is not satisfactory. A method in which a reaction product of a coat-forming organic resin such as an epoxy resin with a hydrazine derivative is used for a coat is proposed in Japanese Patent Application Laid-Open No. 119879/2000 as a method for solving these problems. In this method, an epoxy resin surely shows a relatively high oxygen barrier property as compared with those of the other thermosetting resins such as a urethane resin and an acryl resin, but the level thereof is not satisfactory, and required is a rust preventive and corrosion preventive method for steel plates and steel products in which further excellent corrosion preventive property is exhibited.